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After Nothing

  1. Dec 17, 2003 #1
    What happens to all the Nothing in a vaccum when you fill the vaccum with something?
  2. jcsd
  3. Dec 17, 2003 #2


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    Uh oh. It's "nothing II: the return of nothing!" Amazing how 22 pages of posts can be generated ex nihilo.
  4. Dec 17, 2003 #3
    Why, nothing of course.
  5. Dec 17, 2003 #4
  6. Dec 17, 2003 #5


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    Don't worry. As long as this is just about vacuums, the sheer idiocy of the other thread should be avoided. In answer to the question, it would be appear that there is no such thing as a complete vacuum. Since empty space does not exist, one can't ask what happens to it when it gets filled.
  7. Dec 17, 2003 #6
    Stating it like this is confusing unless you also mention you're counting the metric as a "thing" rather than just as a part of space. (That is what you're doing, correct?)
  8. Dec 17, 2003 #7


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    Yes, though I don't know why matter should have some kind of different status as a "thing".

    I should also mention there are other "things" in space such as gravitational waves and all that quantum stuff that also rule out the existence of a perfect vacuum.
    Last edited: Dec 17, 2003
  9. Dec 17, 2003 #8
    I think it's more natural to see the distance between two points (or something defined locally that defines such distances) as a property of space (or as a part of what "space" means), rather than as a thing in space. It coheres better with descriptions of space as being curved as well as with the phrase "vacuum solution" (of GR).

    True, but do these make a vacuum impossible, or just impossible in our universe?
  10. Dec 17, 2003 #9


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    Well the problem seems to be that the field defines space itself, rather than being something in space. We could take vacuum to mean merely the absence of matter, but then the question seems to lose it's importance. In any event, the traditional view of a complete vacuum (ie. the static, fixed background of the universe)does not seem to have any room in the real world.

    Right, it just seems impossible in the universe we live in. We can easily imagine a vacuum Euclidean universe where nothing happens, but I don't think there is any reason to believe in it. The simplest explanation seems to be that space has no independent existence of gravitational field and so a true vacuum doesn't exist.
  11. Dec 17, 2003 #10
    I've heard that you can reformulate general relativity that way. I don't know whether the loopy formulation is simpler than the standard formulation, though. (Do all those in the know agree that it is, or is this controversial?)

    But isn't that just because you decided to call something "the gravitational field" which we used to call "space"? As I understand it, in classical mechanics or special relativity no one calls it "the gravitational field", and what we've learned in GR is that space(time) is actually curved, and this is what causes (or constitutes) gravity. Intuitively, I think of a vacuum as something that still has distances in it, rather than something formless and distanceless. I don't have any problems with curved vacua.
    Last edited: Dec 17, 2003
  12. Dec 17, 2003 #11


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    I don't know, maybe someone like Marcus could provide some insight on this. Though even without the loop representation it would seem that what we call the gravitational field and spacetime are essentially the same thing. If the metric were to somehow vanish, then distance between points would shrink to zero and the manifold itself would vanish as well.

    Yes, both descriptions are equivalent. You can just as well say that spacetime is just the gravitational field or that the field is nothing but curved spacetime. However, the point to stress is that this curved spacetime (and hence space as well) or gravitational field is a definite something rather than the nothing our intuition makes us believe space is. The modern picture of "space" differs from the classic notion of vacuum, mainly in that that even empty space is not the fixed, static background of Newton.

    That works, but the geometry of this vacuum is always evolving and that greatly contrasts to the vacuum of old.
  13. Dec 17, 2003 #12
    If you made the metric zero everywhere (though I don't think that's allowed), then distances would become zero. If you removed the metric completely, distances would be undefined, and only the manifold would still be left. I think it's the second case that's relevant here.

    Fair enough. The only issue left now is what choice of words to make. I would say that empty space is a "something" both in relativity and in Newtonian physics, only in Newtonian physics it's not a very interesting "something". In relativity it's a dynamical entity, in Newtonian physics it's an absolute background (a dynamical entity with rather boring dynamics).

    We agree that space as a "nothing" does not exist, I think.
  14. Dec 17, 2003 #13
    Doesn't it just... move to where the something was before you put it there? (Ignoring all the controversy on the nature of vacuums and spacetime/gravity, that seems to be the obvious answer, no?)
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